1. Field of the Invention
The present invention relates to a semiconductor device array substrate and a method of fabricating a semiconductor device array substrate, especially a thin film transistor (TFT) array substrate and a method of fabricating a TFT array substrate.
2. Description of Related Art
In recent years, with the progressive manufacturing techniques in optoelectronic and semiconductor fields, flat panel displays are growing rapidly, wherein liquid crystal displays (LCDs) have gradually replaced conventional cathode ray tube displays and have become the mainstream due to its low operating voltage, no radiation, light weight and small volume.
Generally, LCDs are classified into amorphous silicon TFT LCDs and low temperature poly-silicon TFT LCDs. When compared to amorphous silicon TFTs, low temperature poly-silicon TFTs have higher electron mobility (about 100 to 1000 times than that of amorphous silicon TFT). Therefore in addition to being used as pixel electrodes, poly-silicon TFTs can be applied to peripheral circuit regions to be used as circuits that drive LCDs.
In practice, TFTs that are used as pixel electrodes and TFTs that drive circuits require different properties. Generally, TFTs that are used as pixel electrodes have higher demands on uniformity of electrical properties, while TFTs that driver circuits require electrical properties of carrier mobility and high reliability. Device characteristics of TFTs are related to crystal forms and crystal positions of poly-silicon films in the TFTs, and the crystal forms of poly-silicon layers are controlled according to different processes. In recent years, low temperature poly-silicon thin film crystallization methods have been extensively studied, wherein excimer laser crystallization is the mainstream crystallization method.
In order to obtain TFTs of excellent device characteristics, a thin beam directional X'tallization (TDX) process has been proposed, which mainly uses a substrate carrier moving within a sub-micro range and a high-precision optical system added to an original excimer laser system, and uses the following two methods: (1) using the size of a mask slit to pattern a laser beam, so that an amorphous silicon region irradiated with the laser beam starts to crystallize from two sides to the center, and before the main crystal boundary of the irradiated region has not formed yet, moving the mask within a single scan pitch wherein the scan pitch is not greater than half the region of the mask slit. (2) Extending the length and narrowing the width of the laser beam, so that the amorphous silicon region irradiated with the laser beam starts to crystallize from two sides of the short axis to the center transversely, and before the main crystal boundary of the irradiated region has not formed yet, moving the substrate carrier within a single scan pitch wherein the scan pitch is not greater than half the wide region of the short axis of the laser beam. By repeating the above steps (1) or (2), a region of transverse crystallization on the thin film is controlled, and poly-silicon grains grow continuously and do not form main grain boundaries, so that a poly-silicon thin film of high crystal quality is obtained, wherein the size of the crystal grains of the poly-silicon is larger than that obtained by conventional excimer laser crystallization.
However, in the above TDX laser crystallization technique, the laser beam irradiates the amorphous silicon layer disposed on the substrate, and when the substrate is moved during scan operations of the TDX laser crystallization method, the substrate can only be moved within half the region of substrate irradiated by the laser beam. Therefore, when performing scan operations of the TDX laser crystallization method along a direction, not only more laser shots are required, the substrate is also moved more times totally. Hence, although a poly-silicon film of high quality is obtained, the process time is increased so that enhancement in productivity is unfavorable. In addition, the substrate or the mask is moved a single scan pitch. Although the poly-silicon in different regions on the substrate are the same, more laser shots are required, so the total times of moving the substrate or the mask are increased. Hence, although a poly-silicon film of uniform quality is obtained, which means that the main crystal boundaries of the poly-silicon on all positions of the substrate are uniformly distributed, the process time on the substrate is increased so that enhancement in productivity is unfavorable.